Hi, thanks for trying it out!
There are two things that I can imagine going wrong. Maybe it's somehow breaking based on the path you are running the executable in. Is there maybe something like Unicode in there? It should be able to handle whitespaces, but I haven't tested with much else, so if it just doesn't show up, that could be one thing to try, running it from a folder without special characters.
Another thing could be that the executables aren't working on your machine (the ones packaging the game in /vita/external/vita-sdk/). I got a report by someone that they weren't working for him in 1.4, so I replaced them in 1.5, maybe that could be causing issues? Does the 1.4 version work for you? (From here, there's a Dolmexica Infinite Version 1 4 Make-your-own-game kit for Vita download, it's the third one, their names seem to be cut off: https://captaindreamcast.itch.io/dolmexica-infinite-archive) That way I could at least rule out that it's the changes to the packaging that broke anything. Of course it could also be related to them not working on your machine in either 1.4 or 1.5. Is there an error message when you just click on \vita\external\vita-sdk\vita-mksfoex.exe and vita-pack-vpk.exe? Something like "Can't run on this architecture"? I would assume it also pops up when trying to run it from the script, but just to be sure.
Final question would be if it only happens when you try packaging after adding your custom character. Does it also not show up if you just try to package it with the asset folder that comes when you download Dolmexica? If it's that, it might again be some strange filename interfering with the packaging process.
If it's not any of that, my final suggestion would be to open the script (make_vita.bat) and try running the steps of the VPK creation in that script step-by-step on the windows terminal, to check if there are errors popping up there. Although I think if you already tried running the script, it would look similar to how it looked there.
Those are the things I can imagine could still be causing issues, please let me know if none of them work out, I'll also let you know if I come up with anything else. Thanks!